GREENSTONE, in geology, an old term used by many of the earlier writers to indicate fine-grained dark coloured and often considerably decomposed and altered basic igneous rocks, either intrutive or extrusive. It is still—like "felsite" (q.v.)—a useful word in descriptive field geology, since it indicates the appearance and general character of the rock, but does not imply anything definite as to its exact nature and composition, which may be de termined later by laboratory methods.